how to set camera fov and focal lens independently?
The field of view is determined largely by two factors: the lens length and the sensor size (we're going to leave focus distance out of this calculation to keep it simple). So changing the lens size will always change the field of view, unless the sensor size is changed accordingly to compensate.

Increasing the length on the lens will make for a smaller field of view (like a telephoto in a camera). A shorter lens would have the opposite effect, the field of view would be much wider (like in a wide angle lens). How much will depend on the sensor size.

In my opinion, the simplest answer for what you need would be that if you know the field of view you really don't need anything else. Just set it to 69.9 and don't worry about the sensor or lens size.

Knowing the sensor size from the manufacturer might not be the answer either, we don't know many things: how much of the sensor is actually used? Are the measurements of the sensor (and field of view) you are given for the width, height or diagonal. What is the aspect ratio of the generated image? Also there are many things in the real world that do not meet the specifications of the manufacturers (is the 4mm lens really 4mm?)

If you want to make a simulation (for depth of field reasons maybe)
that uses a 4mm lens covering a field of vision of 69.9 degree, then move the sensor width until you get to the numbers you want (5.591mm)



Notice that the field of view you just set up is only good for the Camera view.